1. Field of the Invention
This invention relates generally to a semiconductor technique and more particularly to a siloxan polymer insulation film on a semiconductor substrate and a method for forming the film by using a plasma CVD (chemical vapor deposition) apparatus.
2. Description of Related Art
Because of the recent rise in requirements for the large-scale integration of semiconductor devices, a multi-layered wiring technique attracts a great deal of attention. In these multi-layered structures, however, capacitance among individual wires hinders high-speed operations. In order to reduce the capacitance it is necessary to reduce the dielectric constant (relative permittivity) of the insulation film. Thus, various materials having a relatively low dielectric constant have been developed for insulation films.
Conventional silicon oxide films SiOx are produced by a method in which oxygen O2 or nitrogen oxide N2O is added as an oxidizing agent to a silicon material gas such as SiH4 or Si(OC2H5)4 and then processed by heat or plasma energy. Its dielectric constant is about 4.0.
Alternatively, a fluorinated amorphous carbon film has been produced from CxFyHz as a material gas by a plasma CVD method. Its dielectric constant xcex5 is as low as 2.0-2.4.
Another method to reduce the dielectric constant of insulation film has been made by using the good stability of Sixe2x80x94O bond. A silicon-containing organic film is produced from a material gas under low pressure (1 Torr) by the plasma CVD method. The material gas is made from P-TMOS (phenyl trimethoxysilane, formula 1), which is a compound of benzene and silicon, vaporized by a babbling method. The dielectric constant xcex5 of this film is as low as 3.1. 
A further method uses a porous structure made in the film. An insulation film is produced from an inorganic SOG material by a spin-coat method. The dielectric constant xcex5 of the film is as low as 2.3.
However, the above noted approaches have various disadvantages as described below.
First, the fluorinated amorphous carbon film has lower thermal stability (370xc2x0 C.), poor adhesion with silicon-containing materials and also lower mechanical strength. The lower thermal stability leads to damage under high temperatures such as over 400xc2x0 C. Poor adhesion may cause the film to peel off easily. Further, the lower mechanical strength can jeopardize wiring materials.
Oligomers that are polymerized using P-TMOS molecules do not form a linear structure in the vapor phase, such as a siloxane structure, because the P-TMOS molecule has three Oxe2x80x94CH3 bonds. The oligomers having no linear structure cannot form a porous structure on a Si substrate, i.e., the density of the deposited film cannot be reduced. As a result, the dielectric constant of the film cannot be reduced to a desired degree.
In this regard, the babbling method means a method wherein vapor of a liquid material, which is obtained by having a carrier gas such as argon gas pass through the material, is introduced into a reaction chamber with the carrier gas. This method generally requires a large amount of a carrier gas in order to cause the material gas to flow. As a result, the material gas cannot stay in the reaction chamber for a sufficient length of time to cause polymerization in a vapor phase.
Further, the SOG insulation film of the spin-coat method has a problem in that the material cannot be applied onto the silicon substrate evenly and another problem in which a cure system after the coating process is costly.
According to one aspect of the present invention, a high quality siloxan polymer can be formed by vaporizing a silicon-containing hydrocarbon compound of the formula Sixcex1Oxcex1xe2x88x921R2xcex1xe2x88x92xcex2+2(OCnH2n+1)xcex2 wherein xcex1 is an integer of 1-3, xcex2 is 2, n is an integer of 1-3, and R is C1-6 hydrocarbon attached to Si, and then introducing the vaporized compound with an oxidizing agent to the reaction chamber of a plasma CVD apparatus. The residence time of the source gas is lengthened by reducing the total flow of the reaction gas, in such a way as to form a siloxan polymer film having a micropore porous structure with low dielectric constant.
In the above, if the additive gas does not include an oxidizing agent but includes gases such as He, H2, CH4, etc., a quality low dielectric constant film with a low dielectric constant of k=2.6xcx9c3.1 can still be obtained. If an oxidizing agent is added to the additive gas especially when the compound of the material gas has two alkoxyl groups, it is possible to form a low dielectric constant (xe2x80x9clow-kxe2x80x9d) film with a dielectric constant of k less than 3.1 at low cost by improving productivity by accelerating the film-forming speed. Additionally, in the above, by controlling the flow of an oxidizing agent, an oxygen concentration in the film increases without forming an oxide film structure, and surprisingly, the dielectric constant becomes low, and further, the deposition speed significantly increases. The above effects can significantly be exhibited when (i) the flow rate of the reaction gas is prolonged, (ii) the material gas has two alkoxyl groups, and (ii) an oxidizing gas is added to an additive gas. The resulting siloxan polymer film can have a dielectric constant of 3.1 or lower and have xe2x80x94SiR2Oxe2x80x94 repeating structural units with a C atom concentration of 20% or less (i.e., the compound is fragmented predominantly or selectively at the bond between the hydrocarbon and the oxygen). When the C atom concentration is low, selectivity with etching resist (photosensitive resin) is improved. Additionally, the siloxan polymer has high thermal stability and high humidity-resistance on a semiconductor substrate. Furthermore, because this technique can lower a dielectric constant of a film to approximately 2.4, the scope of applicable devices expands. In addition, according to the present invention, device-manufacturing costs can be reduced and the yield rate can be improved significantly.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.